PHYTOPLANKTON OF THE WEDDELL SEA AREA loi 



ceros neglectus and a further decrease in the proportion of the large forms found domi- 

 nant farther north. This evidently represented the first stage in the transition to yet 

 another distinctive phytoplankton, as at the next two stations Ch. neglectus became 

 dominant, being present in such numbers as to defy adequate estimation. Of the forms 

 of which good counts could be obtained Nitzschia sertata, N. closterium and Fragilaria 

 antarctica were the most abundant in that order of importance, and Corethron valdiviae 

 was the most numerous of the larger forms, though by no means so important relatively 

 as in the richer hauls farther north. As will be gathered from the high percentage of 

 other forms, certain species of less constant occurrence than those tabulated reached 

 a comparatively high degree of importance at these two stations. Chief among these 

 were Dactyliosolen antarcticus (7-4 per cent of the total), Synedra spathulata (47 per 

 cent), Rhizosolenia spp. other than styliformis (3-9 per cent) and Chaetoceros dichaeta 

 (3-4 per cent) at St. WS 550, and the same species to a lesser extent at St. WS 551. 

 At the most southerly station of all (St. WS 552 a-d), worked at the edge of the pack 

 within about 100 miles of the Antarctic mainland, the phytoplankton was very 

 similar, with the exceptions that Ch. socialis made its only appearance amongst the 

 material collected during this cruise, and that the minute species Nitzschia closterium 

 was of greater relative importance. The hauls at this station were also consistently 

 smaller than those obtained in the ice-free water slightly farther north. Clear counts of 

 the two minute species of Chaetoceros could not be obtained, but they were certainly 

 the dominant forms, followed by Nitzschia seriata, N. closterium, Fragilaria antarctica, 

 and Corethron valdiviae in that order of importance. The occurrence of Nitzschia 

 closterium in bulk at this station is of interest, as it bears out the view already expressed 

 that in the south it is only found in numbers in the vicinity of the Antarctic mainland, 

 or of pack-ice. Mr J. W. S. Marr has informed me that he has observed it close in to 

 the land at several points round Antarctica while serving with Sir Douglas Mawson, 

 particularly in the neighbourhood of Enderby Land. 



This phytoplankton found far south near the eastern side of the Weddell Sea bears a 

 close general resemblance to that found during the same season in corresponding lati- 

 tudes in the Bellingshausen Sea, as will be seen from the description of the material 

 from that locality described later (p. 153). One important difference should be noted, 

 however. In the Bellingshausen Sea Thalassiothrix antarctica was found in large num- 

 bers far to the south-west, whereas it was absent from the more southerly catches on the 

 Weddell Sea side. In the Weddell Sea the allied and closely similar form Synedra 

 spathulata was found in moderate numbers in the more southerly hauls, but it was of 

 rare occurrence and little importance in corresponding latitudes in the Bellingshausen 

 Sea. This important difference will be referred to again later. 



The main features of the phytoplankton observations during this cruise are illustrated 

 in Figs. 47 and 48, which indicate the total estimated numbers of phytoplankton organ- 

 isms, and the dominant species at each station respectively. From these it will be seen 

 that seven well-defined types of phytoplankton were encountered. 



Proceeding south-eastwards from South Georgia, an extremely rich phytoplankton 



